Re: [Linux-cluster] Service Group Dependency

From: Lon Hohberger <lhh redhat com>

To: linux clustering <linux-cluster redhat com>

Subject: Re: [Linux-cluster] Service Group Dependency

Date: Fri, 14 Nov 2008 17:11:31 -0500

On Thu, 2008-11-13 at 15:28 +0100, Mauro Casiraghi wrote:
> If i have two or more services how can set dependency for the services
> using Redhat Cluster Suite??
>
> For example if i have service A B end C how can set those dependency:
>
> B depend from A and C depend from B
>
> In this case to stop service group A i must stop before C and B
>
> What is the cluster.conf setting ??
That depends. Starting services is done in the "right" order internally
using the 'depend=' service attribute, but it's done backwards for
'stop' (e.g. stopping A will cause 'dependent' services to subsequently
stop, not stop before A).
Rgmanager has an event-action processor you can use to make this work,
(it's not a typical dependency engine, though). You'd have to:
(a) codify the actions necessary
(b) enable central_processing
(c) set up an event trigger to catch an event *before* a service
transitions and make some decisions.
A-+->B
|
+->C
...should be easy...
A->B->C
...would be more difficult.
There's a (very) experimental dependency engine / transition
calculator / etc. in the master branch of git which uses an A* search,
but it's not likely that we will be integrating it...
-- Lon